The present invention relates generally to optical filtering and, in particular, to a filter exhibiting a reverse photopic response, and means for supporting such filters for use during prolonged surgical and diagnostic procedures, in particular.
The human eye has two classes of receptors: cones and rods. These are located on the innermost membrane of the eye, called the retina, on which objects are imaged. The cones in each eye number between six and seven million. They are located primarily in the central portion of the retina called the fovea, and are highly sensitive to color. Humans can resolve fine details with these cones largely because each one is connected to its own nerve end. Cone vision is known as photopic or bright-light vision.
The number of rods is much larger, on the order of 75 to 100 million, and are distributed over the retinal surface. The larger area of distribution and the fact that several rods are connected to a single nerve end reduces the amount of detail discernible by the receptors. Rods serve to provide a general, overall picture of the field of view. They are not involved in color vision, but are sensitive to low level of illumination. In dim light only the rods are stimulated and objects appear colorless. This characteristic is known as scotopic vision.
As a result of extensive tests with hundreds of observers, the primary colors have been standardized by the Commission International d""Eclairage (CIE) based on the principle of color matching in colorimetry. The CIE primaries are: red (700 nm), green (546.1 nm), and blue (435.8 nm). These primaries produce the maximum response to the eye compared to other wavelengths.
Color vision depends greatly on the illumination. Colors are better resolved in a brightly illuminated scene. Certain colors will produce a higher sensation of brightness than others. This is indicated in FIG. 1, which shows the relative luminosity curve for spectral colors radiated at a constant energy level. As can be from this figure, the human eye is most sensitive to green at about a 555-nm wavelength.
However, the dominant colors associated with most surgical observations are red, blue, and yellow. A limited number of internal organs incorporate the color green. As a consequence, due to the fact that human eyes are most sensitive over a spectral range of 525 nm to 575 nm, in many surgical and diagnostic/examination procedures, physicians must resort to very bright illumination to visualize details. This use of bright light increases eye strain in conjunction with such prolonged procedures. Clearly any advance which would allow a lesser degree of illumination or reduce the attendant eye strain would be welcome by the medical/surgical community, in particular.
The present invention resides in viewing apparatus, particularly for medical and surgical practitioners. Broadly, and in general terms, the invention provides an optical filter incorporating means for achieving a reverse photopic response, such that green light is at least partially attenuated, whereas red and blue light are both substantially transmitted. Since, in the human body, internal organs, blood vessels, and so forth, are primarily red or blue as opposed to green, a practitioner viewing a patient through such a filter may do so using a reduced level of illumination, thereby alleviating eye strain and increasing image contrast. That is, by virtue of the invention, with one or more shade of green removed or attenuated from a medical view containing mostly red and/or blue, either the red colors, the blue colors, or both, become more pure, thereby increasing the contrast therebetween.
The invention includes means for supporting one or more optical filters in the practitioner""s field of view, each filter exhibiting a reverse photopic response. In one embodiment, the means for supporting the filters includes a pair of eyeglass frames, whereas, in an alternative embodiment, the means for supporting the filters forms part of an endoscope for use, for example, in conjunction with laparoscopic surgery. Various technologies may be employed to realize a desired reverse photopic response, including absorptive dyes, dielectric layers, and/or holograms.